Can You Use Two Identical BiDi SFP Modules Together?

In most common single-fiber Ethernet links, two identical BiDi SFP modules will not work together. If both modules transmit at the same wavelength and listen at the same other wavelength, neither receiver sees the signal it is designed to receive.

You normally need a complementary pair. For example, a module marked TX1310nm/RX1550nm needs an opposite module marked TX1550nm/RX1310nm.

The modules must also match in data application, line rate, reach class, connector and fiber type. Both host ports must support the required electrical mode and module coding.

“A/B,” “U/D” and colored labels can help, but the safest method is to compare the actual TX and RX wavelength specifications.

Two Identical BiDi SFP Modules: Quick Answer

Local module	Remote module	Wavelength result	Expected outcome
TX1310/RX1550	TX1310/RX1550	Both transmit 1310; both listen for 1550	No normal link
TX1310/RX1550	TX1550/RX1310	Each TX matches the other RX	Wavelength pairing is correct
TX1270/RX1330	TX1330/RX1270	Each TX matches the other RX	Wavelength pairing is correct
TX1310/RX1550 at 1G	TX1550/RX1310 at 2.5G	Wavelengths complement, rates do not	No normal link
Complementary wavelengths but different power classes	Wavelengths may match	Optical budget may be invalid	Verify exact specifications

Correct wavelengths are necessary but not sufficient. A real link also depends on host SerDes mode, application rate, module coding, fiber, connector condition and received power.

How a BiDi SFP Uses One Fiber Strand

A normal duplex optical link uses one fiber for each direction. A single-fiber BiDi module combines its transmitter and receiver onto one fiber through an internal wavelength-selective optical component.

One wavelength travels from Side A to Side B while a different wavelength travels from Side B to Side A.

For one common vendor-specific pair:

Side A transmits at 1310nm and receives at 1550nm.
Side B transmits at 1550nm and receives at 1310nm.

Other products use different wavelength plans. Cisco documents 1000BASE-BX10-U as TX1310/RX1490 and BX10-D as TX1490/RX1310. Its 10G BXU/BXD products use 1270nm and 1330nm in opposite directions. Cisco 1G BiDi specifications, Cisco 10G BiDi specifications

Do not assume that every BiDi pair uses 1310/1550nm. Read the exact datasheet and label.

Why Two Same-Side Modules Usually Fail

Suppose both ends use TX1310/RX1550 modules.

At the local end, the receiver is filtered for 1550nm. The remote module sends 1310nm, so the local receiver does not receive its intended wavelength.

The same mismatch happens in the other direction. Both transmitters send 1310nm, while both receivers wait for 1550nm.

The switch may still:

Detect the module EEPROM
Display the vendor and part number
Report temperature, voltage or TX power
Show that the laser is enabled

None of these proves that the remote optical wavelength reaches the correct receiver. Module detection and DDM communication are separate from successful optical traffic.

Does “Identical” Always Mean the Link Cannot Work?

For the common fixed-wavelength BiDi SFPs discussed here, two identical same-side modules normally do not form a link.

However, this should not be turned into a universal statement about every product called “BiDi.” Some advanced transceivers use multiple lanes, tunable wavelengths, automatic role selection or proprietary architectures.

The reliable field rule is:

For a fixed-wavelength single-fiber module, compare TX at one end with RX at the other in both directions.

Do not decide only from the words SFP, SFP+, BiDi, WDM, A, B, U or D.

A/B and U/D Labels Are Not Universal Definitions

Suppliers may use:

Side A and Side B
BX-U and BX-D
Upstream and downstream
WDM-A and WDM-B
Colored pull tabs or labels

These names help identify a matched set, but they are not universal wavelength mappings.

Cisco’s 1G BX10-U/D pair uses 1310nm and 1490nm, while its 10G BXU/D pair uses 1270nm and 1330nm. A third-party 1.25G product may use 1310nm and 1550nm.

Never pair modules only because one says “A” and the other says “B.” Verify both TX/RX wavelength lines, rate, reach and optical specifications.

What Else Must Match Besides Wavelength?

1. Data application and actual line rate

Both modules and both hosts must operate in the same required mode. A complementary 1G module and 2.5G module do not create a working link merely because their wavelengths match.

Physical SFP or SFP+ cage fit does not establish rate compatibility. Check the host SerDes mode, firmware, driver and port setting.

2. Fiber type

The LuLeey modules referenced here are specified for single-mode fiber. Do not substitute multimode fiber based only on connector fit.

3. Connector type and polish

LC simplex and SC simplex are different physical interfaces. UPC and APC end faces must not be directly mated.

4. Reach class and optical budget

Distance labels such as 10km, 20km and 40km describe an intended reach class, not a guarantee for every route.

Check:

Minimum and maximum transmitter output
Receiver sensitivity
Receiver overload or maximum input
Actual fiber, connector and splice loss
Any required minimum attenuation

Do not pair modules solely because both packages say “20km.”

5. Host recognition and coding

Some switches accept general coding; others warn about or reject unsupported transceivers. Verify the exact switch, router, NIC or media-converter model and firmware.

6. Operating environment

Commercial and industrial modules may have different temperature ratings. Match the actual installation environment, not only the wavelength.

How to Read a BiDi Module Label

Consider this example:

1.25G, TX1310nm/RX1550nm, SMF, 20km, LC, DDM

Read it as six separate checks:

1.25G: intended application and host rate.
TX1310nm: wavelength leaving this module.
RX1550nm: wavelength this module expects.
SMF: single-mode fiber requirement.
20km: reach class under its specified budget.
LC and DDM: connector and diagnostic capability.

The opposite module should normally read TX1550nm/RX1310nm while matching the other required conditions.

If the label shows only one wavelength, find the full datasheet. A single marketing wavelength may refer only to the transmitter.

BiDi SFP No-Link Troubleshooting

Step 1: Record both exact part numbers

Do not write only “two 1G BiDi modules.” Record both labels, host models, firmware and configured speeds.

Step 2: Build a two-direction wavelength table

Direction	Transmitter	Receiver	Match?
Local to remote	Local TX	Remote RX	Yes/No
Remote to local	Remote TX	Local RX	Yes/No

Both rows must match.

Step 3: Verify rate and host mode

Check the configured and actual port speeds. Confirm that both hosts provide the mode required by the modules.

For 2.5G modules, do not assume that an SFP+ cage provides 2.5G. The host chipset or PHY, firmware, driver and port configuration must support that SerDes mode.

Step 4: Check the fiber path

Confirm that the cable is single-mode, reaches the intended endpoint and does not pass through an incompatible splitter, filter or PON network.

A point-to-point Ethernet BiDi pair is not a substitute for a GPON or XGS-PON ONU and OLT pair.

Step 5: Inspect and clean both connectors

Use an inspect-clean-inspect process with tools matching LC or SC and the connector polish. Never look into an active fiber.

Step 6: Read DDM in the correct direction

Compare the remote module’s TX reading with the local module’s RX reading. Repeat in the opposite direction.

An RX value reported as no signal or very low can support the diagnosis, but DDM accuracy depends on the module and host.

Step 7: Compare power with exact limits

If wavelengths and rates are correct, compare RX power with the receiver sensitivity and overload limits. Too little or too much light can prevent reliable operation.

Step 8: Test a verified pair

Use a known-good complementary pair with the correct coding and reach. Change one variable at a time and record the result.

Example with LuLeey BiDi SFP Modules

LuLeey’s 1.25G SC BiDi SFP pair for 10km is listed as a matched 1310/1550nm pair using one single-mode fiber, an SC simplex interface and DDM. Its page states that TX1310/RX1550 must pair with TX1550/RX1310.

For a 2.5G host, LuLeey’s 2.5G SC BiDi SFP for 20km is listed for a 1310/1550nm complementary pair, SC simplex and single-mode fiber.

The 2.5G product page also states that the host must support 2.5G SFP operation. Do not order it for a fixed 1G port or assume every 10G SFP+ port supports native 2.5G.

Before ordering, provide: